Google's Index Page Counts Like a "Rollercoaster"?

A WebmasterWorld thread has dozens of reports that Google's page index counts of sites have been bouncing up and down like a rollercoaster.

By page index site, I assume they mean the number of pages Google reports when you conduct a site:www.seroundtable.com (aka a site command) search on a domain. I personally do not follow the indexed page counts, since I know those numbers are very rounded and considered estimates but huge increases and decreases may represent something about a site.

However, huge increases that happen over and over again over short periods of time may be just be whacky data from Google.

So which is it? Is Google whacky? Is Google stricter some days on what they should index? Is it simply new data centers messing with webmasters? Is it just something SEOs obsess over for no reason? Is there an issue with these guys sites?